1. Field of the Invention
The present invention generally relates to a process for making high sheet resistivity doped semiconductor resistors and, more particularly, to a process for making such resistors characterized by very narrow and controllable width dimensions.
2. Description of the Prior Art
There is an increasing need for doped semiconductor resistors of high sheet resistivity and small dimensions for use in very large scale integrated (VLSI) circuits. One way to achieve high sheet resistivity is to reduce the width dimension of the resistor to less than a micron, e.g., of the order of 200 to 500 nanometers. This approach is preferred where total resistor size is to be kept at a minimum. The major problem to be solved is how such narrow dimensions are to be achieved for a resistor in a controllable and reproducible manner.
Conventional processes for making doped semiconductor resistors employ lithographic masking techniques for delineating the area of the resistor for diffusion or ion implantation. Light, electron beam and X-ray lithography have been used, in turn, as shorter wavelength radiation was resorted to in an effort to increase resolution to achieve the narrow dimensions desired. For reasons of insufficient resolution or undue expense and complexity or unacceptable reproducibility, each of the foreging approaches has been found to be inadequate to meet the VLSI need for high sheet resistivity doped semiconductor resistors.
So called "sidewall" processes for fabricating narrow dimensioned base, emitter and collector reach-through regions of bipolar transistors or source, drain and channel regions of field effect transistors are taught in U.S. Pat. Nos. 4,209,349 and 4,209,350, filed in the names of Irving T. Ho and Jacob Riseman on Nov. 3, 1978 and for making narrow dimensioned vertical insulating regions between conductive layers is disclosed in U.S. Pat. No. 4,234,362 filed in the name of Jacob Riseman on Nov. 3, 1978 and assigned to the present assignee. Briefly, the narrow dimension is determined by the thickness of a layer deposited upon a different underlying layer having horizontal and vertical surfaces. The horizontal portion of the deposited layer is removed by anistropic etching to leave only the vertical portion which then serves either as a mask for diffusion or ion implantation or as a shaped source of impurity in the event that the deposited layer was doped.